1. Field of the Invention
The present invention generally relates to the field of shellfish aquaculture. More specifically, the invention is directed to improved upwellers and upweller systems for raising shellfish seed. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Related Art
Shellfish aquaculture is a rapidly growing field that is increasingly dependent on hatchery-reared shellfish seed. Although hatchery production is relatively straight-forward, small shellfish seed from the hatchery (1 to 3 mm) are delicate and need to be protected from predators until they are strong enough to resist attacks by, for example, crabs. Shellfish seed which are at least 10 to 20 mm are often considered hearty enough to withstand attack from such predators. Therefore, shellfish seed received from a hatchery must undergo an additional protective rearing stage prior to their introduction into a natural habitat. Nearly all of the shellfish seed in this stage of growth (the nursery phase) are reared in upwellers (mostly land-based, but several floating upweller designs have been tried). This nursery phase of shellfish aquaculture (1 to 20 mm) typically occurs in just three to four months of growth and, for a number of reasons, is the most challenging part of growing shellfish.
Most upwellers for shellfish in the nursery phase employ a screen-bottomed container to hold the shellfish seed while water is pumped up through the shellfish bed to bring new water and food to the shellfish. Additionally, this water circulation also serves the purpose of carrying away any waste that may be generated by the shellfish seed.
As the shellfish aquaculture industry has evolved, several designs for upwellers have been developed and tested. These include land-based systems, wherein water is pumped through tanks in which upweller silos are disposed. They also include floating upweller systems, wherein the upweller is partially disposed within an outdoor body of water and the water is either circulated therethrough by harnessing natural currents (tidal-powered floating upwellers) or with the assistance of pumps. However, most of the upwellers currently in use represent small variations from well known upweller designs, are often crude and, invariably, more expensive than necessary.
In recent years, a number of more innovative aquaculture systems have been developed. Two examples of such systems include U.S. Pat. No. 5,438,958 to Ericsson et al., issued Aug. 8, 1995 and U.S. Pat. No. 4,860,690 to De Santo et al., issued Aug. 29, 1989. The disclosures of the Ericsson et al. and De Santo et al. U.S. patents are hereby incorporated by reference. In particular, the shellfish aquaculture system of De Santo et al. represents a significant advance over the prior art in that the invention includes a marina dock which has been modified to incorporate a shellfish aquaculture system which remains unobtrusively contained under the dock. The primary advantage of the De Santo et al. system lies in its dual-use nature, i.e., the docks can be used as usual while the components of the aquaculture system remain accessible for the seeding, maintenance, harvesting and other conventional aquaculture activities.
One significant drawback of the aquaculture system disclosed in De Santo et al. resides in its utilization of tidal-powered baskets (in lieu of upwellers) to rear the shellfish. Such baskets allow the waters in and around the marine dock to flow about the shellfish seed being grown but the flow rate of the water circulating therethrough is necessarily limited by the natural conditions of the ambient environment. This natural flow rate is typically far too inadequate to permit rapid growth in high concentrations of shellfish seed during the nursery phase. Thus, De Santo et al.'s aquaculture system is prone to either low concentrations of shellfish seed or to stunted shellfish growth.
One way to alleviate the deficiencies in the De Santo aquaculture system is to use aquaculture upwellers in lieu of De Santo's baskets. Upwellers typically consist of a silo formed from a hollow cylindrical piece of PVC pipe and a screen assembly permanently affixed (typically glued) to one end of the silo. In the case of floating upwellers, the upweller is partially suspended within a liquid ambient environment such that the screened end of the silo is disposed well below the surface of the liquid. Additionally, the opposite end of the silo extends well above the surface of the liquid and an exit port in the upweller permits water (and the nutrients disposed therein) to be pumped therethrough at an accelerated rate. A high concentration of shellfish seed may, thus, be placed into an upweller without stunting shellfish growth. Accordingly, the use of upwellers in an aquaculture system such as that taught by De Santo et al. has the potential of further advancing the shellfish aquaculture art.
Unfortunately, still other deficiencies associated with the use of upwellers have prevented the development of a truly advanced shellfish aquaculture system. For example, economies of scale dictate that shellfish aquaculture be conducted on as large a scale as possible to maximize the profitability of the undertaking. However, since conventional upwellers utilize hollow-cylindrical silos, transport and storage of such upwellers is a difficult and costly endeavor. Moreover, because the screen assemblies attached to the upweller silos are permanently affixed thereto, repairing damaged upwellers is a difficult and time consuming process.
Still another deficiency associated with upwellers of the nature discussed above is that a shellfish grower must stock far more upwellers than can be actually used at any given time. Shellfish seed growth is maximized when fluid flow through an upweller is maximized. Conventionally, very young seed are first raised in upwellers having relatively fine screens to permit some fluid passage therethrough while preventing the seed from falling through the pores in the screen assembly. As the shellfish seed grow, however, the seed must be transferred into upwellers having coarser screen assemblies to enhance the fluid flow through the upweller. Thus, throughout the nursery phase of the shellfish life cycle, three or more sets of upwellers could be employed to retain the growing shellfish seed. The shellfish upwellers which are not being utilized at any given time must be stored by the shellfish grower. Naturally, the relatively large space required to do so increases costs associated with the aquaculture endeavor and prevents the attainment of large economies of scale.